Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. By a wide margin the most commonly employed photographic emulsions are silver bromide and bromoiodide emulsions. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications. As employed herein the term "high chloride emulsion" refers to a silver halide emulsion containing at least 50 mole percent chloride and less than 5 mole percent iodide, based on total silver.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
The various photographic advantages were associated with achieving high aspect ratio tabular grain emulsions. As herein employed and as normally employed in the art, the term "high aspect ratio tabular grain emulsion" is defined as a photographic emulsion in which tabular grains having a thickness of less than 0.3 .mu.m and an average aspect ratio of greater than 8 account for at least 50 percent of the total grain projected area of emulsion. Aspect ratio is the ratio of tabular grain effective circular diameter (ECD), divided by tabular grain thickness (t).
In reviewing the various components of the high aspect ratio tabular grain emulsion definition it is apparent that the average aspect ratio of an emulsion can be raised by increasing the ECD of the tabular grains while maintaining tabular grain thicknesses up to the 0.3 .mu.m limit. Once the practical value of tabular grain emulsions was appreciated, the average aspect ratios of the emulsions were soon raised by increasing tabular grain ECD's to their useful limits, based on acceptable levels of granularity. In fact, the earliest patents required the tabular grains to have an ECD of at least 0.6 .mu.m. Thus, the most dramatic initial impact of high aspect ratio tabular grain emulsions was in high speed photographic applications--e.g., at or above 1000 ASA speed ratings.
The next, more difficult improvement was realized by increasing the percentage of the total grain projected area accounted for by the tabular grain population. This required developing a better understanding and control of the conditions under which tabular grains were formed, particularly the conditions of nucleation and twin plane formation. Gradually the capability of precipitating emulsions with the desired tabular grain population accounting for much more than 90 percent of the total grain projected area has been realized.
In considering further improvement of high aspect ratio tabular grain emulsions intended for high speed photographic applications and in considering extending their advantages to moderate and slower speed photographic applications, the realization has occurred that maximizing the photographic advantages of high aspect ratio tabular grain emulsions hinges on being able to satisfy tabular grain percent projected area and average aspect ratio requirements with the thinnest possible tabular grain population.
This realization has led to efforts to produce high aspect ratio tabular grain emulsions containing ultrathin tabular grains. By "ultrathin" it is meant that the tabular grains have a thickness of less than 360 {111} crystal lattice planes. The spacing between adjacent {111} AgCl crystal lattice planes is 1.6 .ANG.. Daubendiek et al U.S. Pat. Nos. 4,672,027 and 4,6983,964 report the preparation of ultrathin high aspect ratio tabular grain silver bromide and silver bromoiodide emulsions.
The art has not, prior to this invention, reported the preparation of ultrathin high chloride high aspect ratio tabular grain emulsions or even attempted to prepare such emulsions. The failure to report the preparation of these emulsions can be attributed to the art recognized difficulty in preparing high chloride tabular grain emulsions, even when they are not ultrathin. Further, there is basis for belief that those skilled in the art have been deterred from such an undertaking by a belief that ultrathin high chloride high aspect ratio tabular grain emulsions would lack the stability required for photographic applications.
Although the art has succeeded in preparing high chloride tabular grain emulsions, the inclusion of high levels of chloride as opposed to bromide, alone or in combination with iodide, has been difficult. The basic reason is that tabular grains are produced by incorporating parallel twin planes in grains grown under conditions favoring {111} crystal faces. The most prominent feature of tabular grains are their parallel {111} major crystal faces.
To produce successfully a high chloride tabular grain emulsion two obstacles must be overcome. First, conditions must be found that incorporate parallel twin planes into the grains. Second, the strong propensity of silver chloride to produce {100} crystal faces must be overcome by finding conditions that favor the formation of {111} crystal faces.
Wey U.S. Pat. No. 4,399,215 produced the first silver chloride high aspect ratio (ECD/t&gt;8) tabular grain emulsion. An ammoniacal double-jet precipitation technique was employed. The thicknesses of the tabular grains were high compared to contemporaneous silver bromide and bromoiodide tabular grain emulsions because the ammonia thickened the tabular grains. Further, tabular grain geometries sought were significantly degraded when bromide and/or iodide ions were included in the tabular grains early in their formation.
Wey et al U.S. Pat. No. 4,414,306 developed a process for preparing silver chlorobromide emulsions containing up to 40 mole percent chloride based on total silver. This process of preparation has not been successfully extended to high chloride emulsions.
Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) developed a strategy for preparing a high chloride, high aspect ratio tabular grain emulsion with the significant advantage of tolerating significant internal inclusions of the other halides. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with a grain growth modifier having as its function to promote the formation of {111} crystal faces. Adsorbed aminoazaindenes, preferably adenine, and iodide ions were disclosed to be useful grain growth modifiers. The principal disadvantage of this approach has been the necessity of employing a synthetic peptizer as opposed to the gelatino-peptizers almost universally employed in photographic emulsions. The minimum mean tabular grain thicknesses reported by Maskasky I are 0.1 .mu.m (625 {111} crystal lattice planes).
Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II), significantly advanced the state of the art by preparing high chloride tabular grain emulsions capable of tolerating significant bromide and iodide ion inclusions using an aminoazaindene growth modifier and a gelatino-peptizer containing up to 30 micromoles per gram of methionine. Since the methionine content of a gelatino-peptizer, if objectionably high, can be readily reduced by treatment with a strong oxidizing agent (or alkylating agent, King et al U.S. Pat. No. 4,942,120), Maskasky II placed within reach of the art high chloride tabular grain emulsions with significant bromide and iodide ion inclusions prepared starting with conventional and universally available peptizers. A minimum mean tabular grain thickness of 0.13 .mu.m (812 {111} crystal lattice planes) is reported by Maskasky II.
No high chloride high aspect ratio tabular grain emulsion has been prepared having a mean tabular grain thickness of less than 0.1 .mu.m (625 {111} crystal lattice planes). Tufano et al U.S. Pat. No. 4,804,621 in investigating the utility of various di(hydroamino)azines as grain growth modifiers reported in Example 2 the preparation of a high chloride tabular grain emulsion failing to satisfy the &gt;8 criterion of high aspect ration exhibiting a mean tabular grain thickness of 0.062 .mu.m (388 {111} crystal lattice planes), which is a grain thickness somewhat above the maximum grain thickness required to realize ultrathin tabular grains. The remainder of the tabular grain emulsions reported by Tufano et al have substantially increased tabular grain thicknesses, and Tufano et al does not address the formation of ultrathin tabular grains in any aspect ratio range.